The invention relates to an electric motor starter for starting motors rotating at synchronous speed.
The electric motor starter of the invention is particularly suited to be used as a controller of electric motors and in that case to replace an ON/OFF-starter controlling an electric motor. The controller is not, however, suitable for starting motors whose speed of rotation is controlled linearly, such as controlled by an inverter, but it is only intended for motors rotating at a so-called synchronous speed which speed is thus dependent on the frequency of the supplying electrical network.
There are various types of electric motor starters depending on the structure of the electric motor to be controlled and their purpose of use and they comprise an equipment adapted for the electric motor to be started at each time, such as a switching contactor according to the power/current of the motor, and a thermal relay. Starters of a single-speed electric motor, of a two-speed electric motor with two windings, of a dahlander-connected motor, of a star/delta connected motor and the like could be mentioned as various starter and motor types. The different operating modes of motors cause dimensioning and installation problems and connection errors as it is not always exactly known at the beginning of planning what kind of a motor is to be used and how the motor will be logically controlled and which additional devices/components are needed in addition to the motor to make the system to be controlled to operate correctly. Installation, planning, drafting and testing of such an ordinary motor starter built of discrete parts (contactors, thermal relays, time relays, switches, push-button switches, lines, etc.), whereby in more complicated cases the probability of a connection and planning error grows considerably. The typically used components are components controlled by the mains current, such as relays and contactors, in which case the conductors intended for the main voltage and a suitable installation manner have to be used in their supply and control lines.
Because of the above, logic circuits are also used in motor starters. Such starters are known e.g. from U.S. Pat. No. 5,206,572 and GB 2,206,423. They disclose solutions where the logical connection required for controlling the motor is achieved by means of semiconductor circuits. When such constructions are used, the logical connection has to be planned and either built of discrete components or programmed separately for each programmable logic circuit for each different purpose and motor type. When the motor type or the control mode of the motor changes, either the discrete components or the connection between them have to be changed or a new program has to be developed and a program thereof has to be stored in the case of a programmable logic circuit. The motor starters based on the use of logic circuits, described in the references cited, are therefore tailored for the purpose and it is relatively difficult to carry out changes.